


Sunken Treasures

by Askellie



Series: Leviathan AU [2]
Category: Undertale (Video Game)
Genre: Alternate Universe - Merpeople, Blood and Gore, Breeding Kink, Come Marking, Crosstale Sans (Undertale), Crosstale Sans/Dreamtale Nightmare Sans (Undertale), Dreamtale Nightmare Sans (Undertale), Ecto-Tentacles (Undertale), Eye Sex, Feeding Kink, Inflation, Kracken Nightmare, M/M, Shark-Mer Cross, Size Difference, Skull Fucking, Tentacle Sex
Language: English
Status: Completed
Published: 2020-11-29
Updated: 2020-11-29
Packaged: 2021-03-10 05:47:52
Rating: Explicit
Warnings: Graphic Depictions Of Violence
Chapters: 1
Words: 2,855
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/27769282
Author URL: https://archiveofourown.org/users/Askellie/pseuds/Askellie
Summary: [Based on Skumhuu's Leviathan AU]Cross has been an outsider for years, alone and unwanted, until he finds a place with the sweet goldfish mer named Dream and his terrifying brother Nightmare, a kracken of the deep. Nightmare's domain is a graveyard of bones and sunken ships, a warning against those that would try to harm his most precious treasures: his brother and his people.
Relationships: Sans/Sans (Undertale)
Series: Leviathan AU [2]
Series URL: https://archiveofourown.org/series/2152734
Comments: 22
Kudos: 378





	Sunken Treasures

**Author's Note:**

  * For [skumhuu](https://archiveofourown.org/users/skumhuu/gifts).



> Sku comes up with the best AU ideas. If you're not following them on Twitter, I highly recommend doing so because that's where most of the glorious backstory of this fic is sitting, but the basic details are:
> 
> \- [Main plotline](https://twitter.com/skumhuu/status/1332213344137318400?s=20)  
> \- Cross is a fox-shark Mer. Dream is a goldfish Mer. Nightmare is a KRACKEN. He is HUGE. Here's a pic for the [size reference](https://twitter.com/skumhuu/status/1332892124656898050). (He and Dream are still brothers. No I have no idea how that works.)  
> \- Nightmare also has a [tiny form](https://twitter.com/zinyrrr/status/1332707053580664833?s=20), but that doesn't appear in this story, it's just rly cute.  
> \- This is just really shameless size difference porn brought on by the observation that some octopi species breed by cumming in their partners' head and I thought that was RAD.  
> \- A group of sharks is called a Shiver. 8D I did a lot of googling research for this piece, but it's still just very shameless porn.

The Narwhal is old for its kind, its body riddled with the long-healed scars of past fights. Cross can respect that it must have been quite the warrior in its prime, but age has slowed its movements. It’s alone, either abandoned by its pod or perhaps it’s come to the depths of its own accord, ready to take its place in the immense graveyard of Nightmare’s domain. 

Normally Cross would choose smaller, safer prey, but he’s confident of his skills and of the creature’s current weakness. The long blade that Killer mocks him for carrying thrusts smoothly through its soft throat, piercing through its collarbone and severing its airways. Its body gives one violent, wracking spasm before going still. Its eyes cloud over, staring sightlessly into the depths as its body slides off Cross’s sword and begins a slow descent down towards the seabed. It lands with a thump that’s distressingly loud, throwing up a cloud of dusty sand, but very little blood leaks from the new corpse, and Cross is satisfied his kill won’t easily attract other predators who might try to steal his prey.

The only problem, he realises as the adrenaline begins to wear off, will be getting it back to Nightmare’s grotto. The Narwhal is half again as long as he is from skull to tail-tip, and significantly heavier thanks to its thick hide and dense, fleshy body. If it was only a matter of himself, he would simply eat what he needs and leave the rest for scavengers, but the whole point of taking down a bigger and more dangerous quarry is to demonstrate that he can be more than simply a guard. He can help feed the whole shiver instead of simply gathering for himself or, when Dream insists, taking from their stores. 

The most sensible approach might be to split the body into a smaller, more manageable piece, but even though Dust claims there’s few nastier predators in the trench than themselves, carrying a fresh and bleeding chunk of flesh all would leave a dangerous trail towards his new home. He won’t risk it, not even knowing any hopeful hunter would be stupid for challenging Nightmare. Besides, some small part of him wants to take the body intact just to prove what an impressive kill it is. It may have been old, but its tusk is still dangerously sharp, and there’s enough meat to keep them fed for a couple of days at least, even with Horror’s appetite. 

He considers the body, wondering if there’s enough air and buoyancy left inside it that he can drag it along the ocean floor. It’ll be slow going, but if he follows the edge of the trench the trail is smooth enough. He might even find a helpful current so he won’t have to do all the work himself.

Nodding determinedly he glides down to grab the corpse by the tail and gives it an experimental tug...and then a second one, with more focus and effort behind it. Even with the powerful swish of his tail the body only moves forward a tiny increment, and the joints between his arms burn from the exertion. He pauses, panting, reconsidering his approach instead of paying proper attention, which is why he’s completely unprepared for the boom of a voice behind him. 

“Having trouble?”

Cross yelps in shock, twisting around with painful speed, his sword half-drawn. Nightmare leans placidly against the jutting shelf of the trench, chin propped against his palm as he looks down at Cross. Even with most of his body hidden in the deep fissure, it’s utterly bewildering how Cross didn’t sense his approach. There should have been some sign -- the shift of water displaced by his tentacles, the loom of his shadow, or even the blatant glare of his glowing eye beaming out of the gloom. It’s one of the many impossible oddities that Nightmare can move through the water as soft and silent as a cloud of ink, utterly undetectable unless he chooses to make himself known. 

The mortification at his own reaction transitions into an even more acute embarrassment at his inattention. “H-how long have you been there?”

Nightmare’s head tilts. It’s still difficult for Cross to read the emotions on such large, alien features, but he thinks there might be a glint of amusement in the curve of Nightmare’s mouth. “Long enough. Your hunting technique is interesting..but effective.” 

Cross splutters, unsure of how to accept the praise. Impressing the rest of the shiver is one thing, but surely this kill is nothing to Nightmare. He reaches out with his unoccupied hand and casually lifts the Narwhal’s limp body as if it weighs nothing, offering it unexpectedly to Cross.

“Well done,” Nightmare says, the resonance of his voice quaking down the length of Cross’s spine like a stroking finger. “You should enjoy your kill.”

Cross unfurls from his startled posture, tail swaying back and forth as he holds in place, shyly admitting, “It’s to share.”

Nightmare blinks, the bright searchlight of his gaze shuttered for one brief moment before resettling on Cross with a more intense interest. “Then let us share it.”

With one smooth, terrifyingly efficient gesture Nightmare brings forth his other hand and gouges the narwhal from its jawbone to its flukes. It splits the body wide open, tearing right through skin, muscle and bone with a gush of blood that spills over Cross like a surface breeze, heavy and breath-taking. His eyes are full with the grisly feast of colourful organs and soft, sumptuous entrails gathered in the palm of Nightmare’s hand like a captivating coral flower. 

“Take your fill,” he murmurs with just enough sternness that it’s more order than request.

Cross is still unused to the rush of bloody water churning through his gills, the taste filling his mouth and lungs with the insatiable hunger of his kind. The thread of command in Nightmare’s voice seems to rend through all of Cross's reservations as easily as he’d carved through the narwhal’s flesh. Dizzily, Cross lurches forward with arms outstretched to bury themselves in the plush innards of the carcass.

The shocking warmth of its insides feels like the rush of a volcanic vent, the water frothing with heat as he curls his claws into the viscera of its guts. The intestines are already sliced and fragmenting, snaking out from the ruptured belly like the tendrils of an anemone. Cross snatches a strand and shoves it into his mouth, jagged fangs tearing easily through its chewy texture. A shattered rib floats past him, and he snatches that next, gredily tearing away the last stripts of meat and sinew before cracking open the bone to guzzle the marrow.

It’s a richer, heavier meal than the sweet surface fishes and mollusks that made up his diet before coming to the deep. His claws dig in, seeking out the tender chunks of liver and blood-rich nodules of kidney. The hunger that drives him is mindless, the desperate surge of instincts he still barely understands. It’s only when the bloody haze in the water starts to dissipate that he can finally bring himself to pull away with a gasp. He pedals reluctantly back from the corpse, wiping ineffectually at his mouth with hands that are equally slicked with gore all the way up to his elbows. 

“Did you have enough?” Nightmare asks, his large eye narrowed with suspicion. “Even Dream would eat more.”

Cross nearly scoffs at that thought; unable to picture Dream gorging himself unreservedly on a carcass like Cross just did. “I’m fine.”

It already feels like too much, his belly more full and satisfied than he normally allows himself. Too much food makes him feel heavy and slow, too sleepy and sated to stay alert. For a lone mer that could be a death sentence, even in the less dangerous waters near the surface. 

That kind of lazy satiety is regularly indulged by the others. They return from their hunts glutted on meat and pride to sleepily drape themselves across Nightmare’s body like suckling remora, basking and bloated from their feasting. Of course Cross feels compelled to stand guard when they’ve made themselves useless, though there’s no imaginable threat that could breach past Nightmare’s ever-watchful gaze.

But that’s not for him, he couldn’t possibly-

“You’re not alone any more,” Nightmare says as if he can effortlessly pluck the thoughts from Cross’s mind. For a creature such as him, it's perhaps not too far-fetched as possibilities go, but more likely it's simply the shameful openness of Cross’s face that makes him easy to read. “You’re mine now.”

He states it as if it's a fact as simple as the blueness of the sky and not the kind of irrevocable promise that hits Cross right where he’s vulnerable. He thought he was well past the point where he was of worth to anyone enough to be deserving of a place among others. He never expected to be more than an outsider, unwanted, alone, but the way Nightmare looks at him now makes him feel like one of the coveted treasures decorating the grotto. 

With more precision and dexterity that should be possible for his size, Nightmare reaches into the shredded carcass and deftly plucks out its heart. The powerful organ is almost the size of Cross’s skull, dark red and saturated with the blood it used to pump. He sets the body carelessly aside as if all that precious meat is simply inconsequential, but before Cross can object he finds himself gently scooped up in Nightmare’s palm and brought close to bear his inescapable scrutiny.

Instinctively, Cross’s body goes rigid from the ineffable knowledge that Nightmare could crush him with a negligent squeeze. His sockets are wide, and his chest feels tight with apprehension, but Nightmare has always shown perfect control of his strength. He feels secured, supported, though held inescapably held in place as Nightmare presents him with the heart.

“Eat,” he says firmly. “I have you.”

Nightmare pressed the flesh against his teeth, and Cross can’t find it in him to refuse. He bites down, teeth sinking into the hearty muscle, releasing a fresh gush of blood across his palate. It’s not the most tender cut of offal, but the heart is nutrient-rich and for shark-mers they’re often gifted as offerings of strength and wellness. He can’t be sure if Nightmare fully understands the underlying meaning but the fact is no one has given Cross a heart since before his time with Gaster, and he’s simultaneously overwhelmed with fragile gratitude and ravenous need.

His tail thrashes with the intensity of his gorging, fingers clawing at Nightmare’s palm as he reverently devours the organ. Every mouthful makes his belly stretch with warning, eating past the point of fullness to exhilarating excess until there’s nothing left but smears of clotted gore on Nightmare’s phalanges. Unthinking, Cross begins to lick them clean, his ribs and gills rumbling softly with contentment and small chirrups of satisfaction catching in his throat. 

His whole body feels warm, tingling with the euphoria of a good feeding. The heaviness in his limbs might be uncomfortable if not for the way Nightmare is cradling him. A tentacle spirals up from the trench, twining around Cross’s tail and settling over the now visibly swollen bump of his stomach. 

“You look like you’re carrying a clutch,” Nightmare observes appreciatively. His suckers gently pluck against Cross’s rough skin like a cascade of affectionate kisses, soothing some of the tenderness from overindulging. Cross’s soft warbling hitches into a lower, shaky sound. A moan. He feels oversensitive and drowsy. The careful stroking feels almost unbearably good, sending little shudders down the long length of his spine all the way to the bladed tip of his tail.

Blood drunk and uninhibited, he arches into the contact, pelvic fins fluttering tremulously. “N-night…”

“Shhh,” Nightmare soothes him. For the first time, Cross is close enough to see that the uncanny cyan of his eyelight isn’t just a single orb of color. There’s subtle patterns shimmering in his socket, swirls and strokes of hypnotising variance that look like the scrawlings Cross has sometimes seen in ancient caves. Words of an unfamiliar language snaking around in his socket like mesmerising poetry. “I have you. I take care of what’s mine.”

More tentacles slither over his hand, curling over Cross like the arms of a lover. They tangle around his hands and wrists like tangling vines. Another skims around his throat, delving into the delicate spaces between the vertebrae. The extra restraint proves necessary a moment later when Cross feels the slippery glide of suckers tracing the still healing harpoon scar across his abdomen. There’s no pain at the pressure, not after all the careful tending and healing Dream and Nightmare had poured into him, but it’s shockingly tender still and he bucks wildly with a hitching gasp of surprise. The sharp movement makes his belly give a painful twinge of objection, but the faint hurt is easily drowned out by the rising pleasure.

Yet another indulgence Cross has denied himself except during the unavoidable onset of mating season. The brief, clumsy acts of desperately bringing himself to release hidden in rough, cramped crevices for privacy and safety don’t compare to the unrestrained generosity of Nightmare’s attention. His tentacles envelop Cross’s much smaller body, eagerly searching out all the places that make him whimper and writhe, teasing at the delicate joints of his fins and the now flushed and straining cleft of his sex. A coaxing tendril slides up the length of his sensitised slit, and with a helpless shudder Cross surrenders to its demand and lets his hemipenes slide out into the open, already engorged and throbbing from the building pressure in his core. 

“You’re so sensitive when you’re full,” Nightmare observes. His luminous gaze feels as constrictive as the pressure of the lower depths, and for a moment Cross feels a stab of anxiety at being so vulnerable and exposed even through the haze of bliss. Then he feels the smooth glide of Nightmare’s tentacle sliding into the gap between his shafts, stroking them both, and the fear evaporates in a frisson of ecstasy. “Perhaps you can take a little more?”

A new appendage rises up in Cross’s vision, longer and thinner than the other tentacles. Even at a distance, it seems to waft with a shimmer of essence that smells intensely of Nightmare and something undeniably sexual. Cross struggles to swallow the sudden glut of salivation, leaning yearningly towards it as it glides towards his skull.

For a moment, he hesitates on his approach. He doesn’t want to risk the delicate appendage snagging on his jagged teeth but he desperately wants to taste it. He extends his tongue with a pleading keen, and is rewarded with a fleeting brush of the tip smearing its thick, viscous coating before it pulls away. The flavour is indescribable, a dangerous tang that buzzes in his bones like when a lightning strike hits the water. Immediately he craves more, but Nightmare pulls the appendage teasingly out of reach with a rumbling chuckle that seems to shake the ocean floor. 

“Like this,” he says, tracing the rim of Cross's eye socket and -- when there’s no sign of discomfort or rejection -- lets it slide into his skull. “So you can tell me if I need to stop.”

Its a thoughtful gesture, but Cross doesn’t know how effectively he can honor it when there’s nothing in his skull but _want_. His voice feels lost, overwhelmed as the appendage plunges in deeper, curling and coiling around itself in a tangled knot of slippery, lustful intent. It’s utterly overwhelming, unlike anything he’s ever experienced, almost too intense to be pleasurable and yet his body spasms with powerful jolts of pleasure as he loses all sense of himself. 

The first of his hemipenes comes hard, spilling its violent essence into the water as he writhes against Nightmare’s hold. The second takes a little more coaxing, and it’s not until the tip of Nightmare’s tentacle presses against his slit - too big to fit but stretching him in the most excruciatingly pleasurable way -- that he reaches his second climax with a ragged cry. 

Through the resonance of contact and their mingled magics he can feel his pleasure feeding back into Nightmare, and there’s another release, this time in his skull. It feels as hot and intense as when he first plunged his face into the innards of the narwhal. Cross feels overcome, the inside of his skull sticky and heavy but thrumming with Nigthmare’s possessive satisfaction.

 _Mine, mine, mine_ , is the unspoken intent radiating from that essence. _Owned. Loved. Protected._

It’s too profound to really sink in, and Cross is still senseless from all the overstimulation. His body is limp with utter relaxation as he curls trustingly into the nest of tentacles that envelop him like a blanket,

“So good for me,” Nightmare coos, cradling with care. “Let me take you home.”

Cross makes a slurred noise of assent, but he's already mostly asleep, basking in the safety of Nightmare's cherishing hold.


End file.
